Such fluorine-containing glasses have the property of presenting a wide window of transparency (wavelengths generally lying in the range 0.3 micrometers to 8 micrometers) and in particular, in the mid-infrared, of presenting ultratransparency considerably greater than that of silica.
The field of application for fluorine-containing glasses can thus extend from long distance telecommunications using very low loss optical fibers to laser fibers for microsurgery, and in general to components for integrated optics.
It is known that the preparation of optical components and fibers requires materials of very high purity. In particular, certain cations such as iron, cobalt, nickel, etc. must not be present at concentrations any greater than 10 to 50 parts per billion (10.sup.9, ppb). Conventional methods of manufacturing pieces made of fluorine-containing glass by melting and by quenching give rise to large amounts of pollution by contact with the receptacles used for melting and for casting. It is therefore highly advantageous to be able to prepare fluorine-containing glasses by vapor deposition in order to avoid such drawbacks.
However, it is difficult to use the usual vapor deposition technique because of the chemical complexity of fluorine-containing vitreous systems. Fluorine-containing glasses generally include at least four constituents. Thus, although the composition of a vapor in equilibrium over a binary mixture can generally be predicted, adding other constituents makes the evaporation more complex and the theoretical ratios of the constituents are no longer obtained in the vapor. In general, the molten mixture in a crucible becomes enriched with one or more constituents that are less volatile than the others, such that the change in the composition of the liquid to be evaporated rapidly departs from the thermal range of the liquidus. Evaporation is then observed to halt and the composition present in the crucible crystallizes.
International patent application WO 90 08 743 shows that it is possible to deposit vapor of fluorine-containing vitreous compositions more easily by melting the constituents of the glass to be deposited in a host bath constituted by a molten mixture of fluorides, itself forming a vitrifiable composition constituted by elements that are less volatile than the fluorides of the glass to be deposited. That technique makes it possible to avoid the difficulties encountered in the direct evaporation method since it has been discovered that the composition of the "host glass" remains substantially constant, or in any event varies sufficiently little for it to be possible to keep the bath in the liquid state.
Nevertheless, there still remains a problem if it is desired to use vapor deposition to obtain a film of fluorine-containing glass on a substrate, where the glass is heavily doped with rare earth. Thus, if fluorides or halides of rare earths are also added to the crucible, it turns out to be impossible to obtain a film having a predetermined composition of rare earths.